1. Field of the Invention
The present invention relates to an intake system for an internal combustion engine having an exhaust gas recirculation system.
2. Description of the Related Art
The temperature in a vehicle engine compartment becomes relative high due to the heat released from an internal combustion engine therein, so that an intake passage in the engine compartment is heated and the temperature of intake air supplied to the combustion chamber becomes relative high. This reduces the charging efficiency and the engine power drops. In particular, when in a high-load driving condition, engine knock (pre-ignition) is apt to occur. To solve these problems, Japanese Unexamined Patent Publication No. 2-153252 discloses an intake passage entirely surrounded by a heat insulator. It is also known that the intake passage can be made of any material having a high heat-insulating ability. These related arts can prevent the intake passage from being heated by the atmosphere in the engine compartment.
In the case where the intake system has an exhaust gas recirculation system, by which a part of exhaust gas is recirculated via the intake passage to the combustion chamber, and the main components of the exhaust gas are inert gases having a high heat capacity, the combustion temperature is reduced by the inert gases and an amount of nitrogen oxides generated by combustion can be decreased. When such exhaust gas recirculation is carried out, the above intake passages, with increased heat-insulating ability cause the temperature of the intake air supplied to the combustion chamber to be increased, in contrast to the normal intake passages. This is because the exhaust gas is very hot so that the temperature of the intake air becomes higher than the atmosphere in the engine compartment when the exhaust gas is mixed therewith, and thereafter, in the normal intake passage where heat should be released from the intake air, the heat is not released via the above intake passage. Moreover, the above intake passage is heated by the intake air mixed with the exhaust gas and remains at a high temperature immediately after the exhaust gas recirculation is stopped, so that high-temperature intake air is also supplied to the combustion chamber at this time. The above problems occur in the case that the exhaust gas recirculation system is connected to the above intake passage.